1. Field of the Invention
The present invention relates to a liquid crystal display, and a manufacturing process of the liquid crystal display.
2. Prior Arts
In the conventional active matrix type liquid crystal displays, a liquid crystal display adopting the twin-stead-nematics display system (hereinafter referred to as "TN display system") is heretofore well known. In the liquid crystal display adopting such a display system, a pair of transparent electrodes for driving a liquid crystal are respectively arranged on opposed surfaces of two transparent insulating substrates opposite to each other, and orientation control of the liquid crystal has been made by establishing direction of an electric field applied to the liquid crystal to be almost perpendicular to the substrate surface.
In the mentioned liquid crystal display adopting the TN display system, the liquid crystal molecule is driven in the longitudinal electric field perpendicular to the transparent insulating substrates. Accordingly, in a state that the liquid crystal molecule has been raised in oblique direction, there is a visual dependency that optical characteristics are different depending upon visual direction. As a result, a problem exists in that contrast in case of changing the visual direction changes largely, and it becomes difficult to display a half tone.
A number of attempts for correcting such a visual dependency have been heretofore proposed, and among them a display system in which direction of electric field is established to be almost in parallel to the substrate surface (hereinafter referred to as "parallel electric field") has come to draw a good deal of attention in the field of art (such a display system is hereinafter referred to as "IPS (In-Plane Switching) system"), from the viewpoint that the visual dependency may be largely corrected and that the manufacturing process in TN display system may be substantially utilized as it is. For example, an IPS type display was reported in the "In-Plane Switching of Nematic Liquid Crystals", R. Kiefer et al., JAPAN DISPLAYS 1992, HIROSHIMA, pp.547 to 550, and thereafter a number of electrode arrangements for putting the IPS system into practical use have been proposed.
For example, FIG. 9 is a schematic sectional view of a liquid crystal display device of an IPS type liquid crystal display shown in "Principles and Characteristics of Electro-optical Behaviour with In-Plane Switching Mode", M. Oh-e et al., ASIA DISPLAY 1995 HAMAMATSU, pp.577 to 580. In the drawing, reference numeral 1 designates a lower transparent insulating substrate on a side where an incident light comes in, numeral 2 designates an upper transparent insulating substrate, numeral 3 designates a liquid crystal layer held between the lower transparent insulating substrate 1 and the upper transparent insulating substrate 2 opposite to each other, numeral 3a is a liquid crystal molecule forming the liquid crystal layer 3, numerals 4 and 5 are respectively a first picture element electrode and a second picture element electrode both formed on the surface side of the lower transparent insulating substrate 1 opposite to the upper transparent insulating substrate 2. A predetermined voltage is applied separately to the first picture element electrode 4 and the second picture element electrode 5, and an amount of torsion of the liquid crystal molecule 3a is changed by the parallel electric field generated between the first picture element electrode 4 and the second picture element electrode 5, whereby an intensity of transmitting light in the liquid crystal layer 3 is controlled.
Further, display luminance is an important factor for evaluating a performance of the liquid crystal display, and it is a recent trend that with increasing demand for liquid crystal display of low power consumption, light quantity of back light is restrained. Therefore, improvement in light transmittance of liquid crystal display device is one of the essential technical problems to be solved.
However, in the IPS type liquid crystal display shown in FIG. 9, because a metal not permitting the visible light to transmit is usually employed as a electrode material forming the first picture element electrode 4, second picture element 5, etc., reduction in luminance is considerable. Because the light transmittance of liquid crystal display is in proportion to a percentage of opening (percentage of a light transmitting portion to the entire area of picture element), the light transmittance may be improved by increasing the percentage of opening. However, such increase in the percentage of opening is subject to restriction from the following viewpoints.
1. There is an upper limit value in the distance between the first picture element electrode 4 and the second picture element electrode 5 in view of maintaining the electric field strength to be a certain value or more and controlling smoothly the liquid crystal molecule 3a.
2. There is an lower limit value in width of the first picture element electrode 4 and the second picture element electrode 5 in view of preventing disconnection.
Taking the above restriction into consideration, it is desirable that the distance between the first picture element electrode 4 and the second picture element electrode 5 is 20 .mu.m or less, and the width of the first picture element electrode 4 and the second picture element electrode 5 is 10 .mu.m or more. When applying these conditions to the electrodes of a liquid crystal display of which picture element size is 300 micron in length and 100 micron in width (liquid crystal display for personal computer recently merchandized), percentage of opening is approximately 25%, which is considerably lower than the percentage of opening (50% or more) of the liquid crystal display of the conventional TN system. As a result, there arises a problem of reduction in light transmittance of the liquid crystal display and reduction in display luminance.
To improve the display luminance of IPS type liquid crystal display, the Japanese Laid-Open Patent Publication No. Hei 7-306399 proposed a method for achieving a high luminance of liquid crystal display in which a microlens array is arranged on the outside of a pair of transparent substrates between which a liquid crystal layer is held, and a light emitted from back light is refracted by the microlens and condensed on the opening portion.
In the conventional IPS type liquid crystal display of above construction proposed by the Japanese Laid-Open Patent Publication No. Hei 7-306399 in which a microlens array is arranged on the outside of a pair of transparent substrates between which a liquid crystal layer is held as a method for achieving a high luminance, there are following problems.
1. Because manufacturing process of the microlens array is complicated, in which the microlens are formed by injecting a molten acrylic resin into a mold and annealing the acrylic resin, special process and treatment are required, which results in a problem of increase in cost.
2. Because the microlens array is arranged on the outside of a pair of transparent substrates between which a liquid crystal layer is held and exposed to the air, there arises a problem of malfunction or failure due to sticking of dust, etc.
3. It is not easy to adjust a focal distance of the microlens array. Though the focal distance is adjusted according to the difference in refractive index between the microlens and the air, because the refractive index of the air is constant, for the purpose of adjusting the focal distance, it is necessary to change the refractive index of the microlens. Moreover, though the focal distance may be changed by changing the radius of curvature of the microlens, for the purpose of forming a microlens of different radius of curvature, it is necessary to prepare another mold, which brings about a further problem in the aspect of cost and work efficiency. In particular, when intending to lay out a design of optimum luminance characteristic using the focal distance as a parameter, it is not practical to employ the microlens disclosed in the Japanese Laid-Open Patent Publication No. Hei 7-306399.